(1) Field of the Invention
The present invention relates to pneumatic tires having improved fatigue resistance of cords used.
(2) Related Art Statement
Heretofore, polyvinyl alcohol based synthetic fibers (hereinafter referred to briefly as "PVA fibers") have widely been used for industrial fibers as rubber reinforcement. However, the PVA fibers have poor fatigue resistance, and are essentially soluble in water. Due to such polymer characteristics, the PVA fibers have inferior hot water resistance. Therefore, it is the current circumstances that as tire-reinforcing cords undergo of frequent applications of bent strains, the PVA fibers are merely used as a part of belt materials in radial tires, which belt materials undergo relatively smaller input strain.
However, as described in Japanese patent application Laid-open No. 59-130,314 and 59-100,710, tenacity of the PVA fibers has come to be enhanced by increasing the molecular weight to an extremely great extent (for instant,the average molecular weight: 400,000 or more). However, it is difficult to industrially produce PVA polymers having such a super high molecular weight. Further, their production cost greatly rises due to their difficult production as compared with fibers of such as polyester or nylon used for general tire-reinforcing cords. Thus, the PVA polymers have not acquired commercially competitive force.
Under these circumstances, a method has been discovered, which allows industrially relatively easy supply of high tenacity PVA fibers in a large scale by increasing the molecular weight of PVA polymers to an extent slightly greater than that of the conventional PVA fibers (e.g., Japanese patent application Laid-open Nos. 60-126,311 and 60-126,312). Thus, there comes a fair prospect that the PVA fibers are industrially and commercially used for tire cords. Although the high tenacity PVA fibers thus supplied have inferior tenacity and modulus as compared with aramid fibers, strength of the former is improved far more greatly than conventional fibers such as of nylon, polyester, etc. Thus, such PVA fibers have first been thought to be sufficiently used as tire cords. As described in Japanese patent application Laid-open No 61-108,713, the high tenacity PVA fibers obtained by the above method have greatly improved resistance against applications of mechanical strains as compared with the conventional ones. Therefore, thus improved PVA fibers have been thought to possess practically sufficient fatigue resistance as tire cords.
However, the present inventors have found that the high tenacity PVA fibers obtained by the above method have serious defect in fatigue resistance. That is, the PVA fibers lack fatigue resistance as tire cords as they are, and their cords are cut during running on general roads (hereinafter referred to as "CBU": cord breaking-up). Thus, the PVA fibers are not at all suitable for practical application from the standpoint of tire safety. This will be explained in more detail.
Passenger car tires having a tire size of 195/70 SR 14 were prepared by using fiber materials shown in the following Table 1 under conditions shown therein as cords for a carcass ply. With respect to those tires, a strength-retaining percentage of the cords of the carcass ply was evaluated after running on drum or after running on road in comparison with strength of fresh cords. Results obtained are also shown in Table 1. The strength-retaining percentage of the cords in the carcass ply was measured at a location of the tire marked by "x" in FIG. 1.
TABLE 1 __________________________________________________________________________ Cord Strength-retaining Strength-retain- twisting percentage after ing percentage Cord condtruction 20,000 km running after 50,000 km construction (turns/10 cm) on drum running on road __________________________________________________________________________ Conventional PVA fibers 1800 d/2 36 .times. 36 CBU at 4,700 km -- High tenacity PVA fibers 1500 d/2 39 .times. 39 60% 20.about.40% (e.g. Japanese patent application laid-open No. 61-108,713) Polyester fibers 1500 d/2 40 .times. 40 60% not less than 90% __________________________________________________________________________
As is clear from Table 1, the cord strength-retaining percentage of the high tenacity PVA fibers after running on drum is equivalent to that of the polyester fibers. On the other hand, the cord strength-retaining percentage of the polyester fibers after running on road was not less than 90%, whereas that of the high tenacity PVA fibers was lowered to 20 to 40%. In addition, some tires developed CBU, and were just ready to be punctured.
The above road running was carried out while a test tire was mounted on an ordinary vehicle and inflated at a normal internal pressure (ordinarily 1.7 kg/cm.sup.2). This running conditions were in a controlled state as tire use conditions. In ordinary use conditions, there is the possibility that tires may be used in an abnormal state, e.g. sometimes overloaded or sometimes run at an internal pressure of not more than 1.0 kg/cm.sup.2. Therefore, that the cord strength-retaining percentage after running 50,000 km on roads in the controlled state was 20 to 40% means that it must be judged that tires do not guarantee at all safety under ordinary use conditions. Thus, it was judged that such tires could not be practically used at all.
As mentioned above, since it was considered that phenomena peculiar to the PVA fibers could not be detected by the tire drum running test or a laboratory running test a so-called tube fatigue test, the present inventors further conducted the following tests to thoroughly investigate causes for the above fatigue. First, folded belt structure passenger car tires having a tire size of P235/75R15 were prepared by using different fiber materials shown in Table 2, respectively, as belt cords under conditions also shown therein. With respect to those tires, strength-retaining percentage of the belt cords after running on roads was evaluated in the manner as mentioned above. Results obtained are also shown in Table 2. The strength-retaining percentage of the belt cord was measured at a location marked by "x" in FIG. 2.
TABLE 2 ______________________________________ Strength- Cord retaining Cord twisting percentage construc- construction after 32,000 km tion (turns/10 cm) running on road ______________________________________ Aramid fiber 1500 d/2 32 .times. 32 95.about.100% High tenacity 1500 d/2 31 .times. 31 60.about.70% PVA fibers (e.g. Japanese patent applica- tion lain-open No. 61-108,713) ______________________________________
As is clear from the above Table 2, even when the high tenacity PVA fibers were used as the belt cords, the cord strength-retaining percentage was lowered to about 60% of that of the fresh cord. Thus, it was revealed that such high tenacity PVA fibers posed a great problem in fatigue resistance.